Abstract [en]

This paper explains the plastic shrinkage and the cracking process of fibre reinforce concretes, based on the settlement, tensile strength, and the mass loss, i.e. weight of the evaporated water. The effect of a type of steel fibres (known as RTSF), obtained through recycling end-of-life tyres, on plastic shrinkage cracking of fresh self-compacting concrete has been investigated. The vertical and the horizontal deformations of the concretes in addition to the amount of the evaporated water have been measured in an unrestrained mould. Another specimen was cast in a restrained mould, from which the crack area was calculated. The outcomes are then compared with the performance of a commercially available hooked steel fibre (HSF). During the experiments, mixtures containing 2.5, 5 and 10 kg/m3 of RTSF or 5 and 7.5 kg/m3 of HSF have been studied. The results show that RTSF and HSF have an almost similar impact on reducing the crack area, especially with 5 kg/m3 of fibres.

In thesis

Sayahi, Faez

Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.

2019 (English)Doctoral thesis, comprehensive summary (Other academic)

Alternative title[sv]

Plastiska krympsprickor I betong : begränsning och modellering

Abstract [en]

Early-age (up to 24 hours after casting) cracking may become problematic in any concrete structure. It can have a negative influence on the aesthetics of the concrete structure, as well as decreasing the durability and serviceability by facilitating the ingress of harmful materials into the concrete bulk. Moreover, these cracks may expand gradually during the member’s service-life due to long-term shrinkage and/or loading. Early-age cracking is caused by two driving forces: 1) plastic shrinkage cracking which is a physical phenomenon and occurs due to rapid and excessive loss of moisture, mainly in form of evaporation, 2) chemical reactions between cement and water which causes autogenous shrinkage. In this PhD project only the former is investigated.

Rapid evaporation from the surface of fresh concrete causes negative pressure, known as capillary pressure, in the pore system. This pressure pulls the solid particles together and decreases the inter-particle distances, causing the whole concrete element to shrink. If this contraction is hindered in any way, the induced tensile stresses may exceed the low tensile strength of the concrete and cracking starts. The phenomenon, occurring shortly after casting while the concrete is still in the plastic stage, is mainly observed in elements with high surface to volume ratio such as slabs and pavements.

Many parameters may affect the probability of plastic shrinkage cracking. Among others, effect of water/cement ratio (w/c), fines, admixtures, geometry of the element, ambient conditions (i.e. temperature, relative humidity, wind velocity and solar radiation), etc. has been investigated previously. In the presented research, in addition to studying the influence of various parameters, i.e. w/c, cement type, coarse aggregate content, superplasticizer dosage, admixtures, and steel fibres, effort is made to reach a better and more comprehensive understanding about the cracking governing mechanism. Evaporation, capillary pressure evolution and hydration rate are particularly investigated in order to identify their relationship.

This project started with extensive literature study which is summarized in Paper I. Then, the main objective was set upon which series of experiments were defined. The utilized methods, material, investigated parameters, and results are presented in Papers II-IV. A model was, then, proposed in Paper V, to estimate the cracking severity of the plastic concrete.

It has been observed that evaporation is the driving force behind the plastic shrinkage crackingin concrete. However, a correlation between evaporation, rate of capillary pressure development and the duration of dormant period governs the severity of the phenomenon. Among other things, the results show that rapid capillary pressure development in the pore network accompanied by slower hydration significantly increases the cracking risk.